Because of the relatively high weight of steel compared to aluminum alloys, steel is used only in those aircraft components where exceptional strength is required. One component conventionally made of ultra high-strength steel (270 to 300+ KSI) is the flaptrack. On the Boeing 737 aircraft, these tracks are additionally stiffened and reinforced by bolting a reinforcing plate along the length of the hockey-stick-shaped flaptrack. The holes for the bolts are workhardened before the plate is attached.
In the past, holes in high-strength steel members were coldworked by predrilling the hole to a size slightly smaller than the finished hole. A lubricant was then applied to the hole surface and baked for several hours or a split-ring sleeve was inserted in the predrilled hole. An oversized mandrel was pulled up through the baked lubricant or split sleeve.
While the results of this process were, for the most part, satisfactory, the cost was believed to be too high. Specially hardened tool steel mandrels were needed to work the ultra high-strength steel holes. However, the hardened mandrels broke frequently under tension, i.e., while they were being pulled through a workpiece, adding significantly to process cost. Furthermore, the split in the pull-up sleeve ring would results in creation of a ridge in the coldworked hole. This ridge had to be cold worked a second time with an oversized mandrel if the ridge was excessive.
Another cost factor was that any substantial deviation in the process, such as a mandrel breaking or a sleeve being misaligned in the hole, required that a hole be reworked to accommodate an oversized bolt.
U.S. Pat. No. 2,185,483 to Ward shows a method of hardening forged or cast holes in manganese steel work pieces by pushing a steel pin through the hole. Australian Patent No. 136,701 shows a method of making tubular wrought metal fixtures by pushing a tapering die member through a supporting seat in the work piece.
U.S Pat. No. 3,270,410 shows the use of a tapered bore in aircraft component parts which are put under permanent compression by inserting a right circular cylindrical fastener in the tapered hole. U.S. Pat. No. 3,434,327 to Speakman shows a method of coldworking holes to exceed the materials elastic limit and then allowing the displaced material to rebound to approximately one-half of the amount the material was originally displaced.
U.S. Pat. No. 4,129,028 shows the use of stress waves to work a hole to a desired shape and finish. U.S. Pat. No. 4,649,728 shows cold forging or cold rolling a tube over its entire length to reduce the outside diameter and maintain the inside diameter, and U.S. Pat. No. 4,771,627 shows stress coining holes to relieve stress concentration in the wall of the hole using a vibrating tool.
While the aforementioned patent publications each relate to working holes in metal workpieces, none teaches or suggests a method that was found to be acceptable in the working of holes in high-strength steel aircraft component parts. Accordingly, applicants developed a novel and cost- effective method of doing so which shall be described in greater detail in the brief summary the several figures and the detailed description which follow.